flow-skill-engineer-hook

name	flow-skill-engineer-hook
description	Creation and configuration of event hooks/plugins to manage agent behavior, command filtering, auditing, and automation. Works across IDEs (Cursor, Claude Code, OpenCode). Use when you need to: (1) Create a new hook (e.g., for formatting or security checks), (2) Configure hooks/plugins, (3) Implement logic for blocking or modifying agent actions via scripts.

Hook / Plugin Creator

Overview

This skill helps design and implement event hooks (or plugins). Hooks allow intercepting agent actions (command execution, file read/write, tool usage) and applying rules: allow, block (with explanation), request confirmation, or modify input data.

IDE Detection and Hook Placement

Hook Paths by IDE

IDE	User Hooks	Project Hooks	Format
Cursor	~/.cursor/hooks.json	.cursor/hooks.json	JSON config + shell scripts
Claude Code	~/.claude/settings.json	.claude/settings.json .claude/settings.local.json	JSON config (4 hook types)
OpenCode	~/.config/opencode/plugins/*.{js,ts}	.opencode/plugins/*.{js,ts} opencode.json plugin (npm)	JS/TS modules (event-based)

Detection Strategy

1. Check for IDE-specific markers:
   • .cursor/ directory → Cursor
   • .claude/ directory → Claude Code
   • .opencode/ directory or opencode.json → OpenCode
2. If multiple detected or none → ask the user

IMPORTANT: After detecting the IDE, read ONLY the corresponding reference file. Do not load all references.

Hook Type Availability Matrix

Type	Cursor	Claude Code	OpenCode
Command (shell script)	Yes	Yes	No (use plugin code)
Prompt (LLM-evaluated)	Yes	Yes (8 events)	No
HTTP (POST to URL)	No	Yes	No
Agent (multi-turn subagent)	No	Yes (8 events)	No
Programmatic (JS/TS code)	No	No	Yes

Cross-IDE Event Mapping

Cursor Event	Claude Code Event	OpenCode Equivalent
beforeShellExecution	PreToolUse (matcher: "Bash")	tool.execute.before
afterShellExecution	PostToolUse (matcher: "Bash")	tool.execute.after
preToolUse	PreToolUse	tool.execute.before
postToolUse	PostToolUse	tool.execute.after
postToolUseFailure	PostToolUseFailure	event → tool.execute.after
sessionStart	SessionStart	event → session.created
sessionEnd	SessionEnd	event → session.deleted
subagentStart	SubagentStart	—
subagentStop	SubagentStop	—
stop	Stop	event → session.idle
preCompact	PreCompact	experimental.session.compacting
afterFileEdit	PostToolUse (matcher: "Edit|Write")	event → file.edited
beforeSubmitPrompt	UserPromptSubmit	chat.message
beforeMCPExecution	PreToolUse (matcher: "mcp__.*")	tool.execute.before
afterMCPExecution	PostToolUse (matcher: "mcp__.*")	tool.execute.after
beforeReadFile	PreToolUse (matcher: "Read")	tool.execute.before
—	PermissionRequest	permission.ask
—	Notification	event → various
—	TeammateIdle	—
—	TaskCompleted	event → todo.updated
—	ConfigChange	—
—	InstructionsLoaded	—
—	WorktreeCreate / WorktreeRemove	—

Main Workflow

1. Detect IDE using markers above
2. Read the IDE-specific reference (see Resources below)
3. Define the Event: Choose from the mapping table above
4. Choose Implementation Type per availability matrix
5. Configure: Add config to the correct location
6. Implement Logic: Create the script/plugin
7. Test: Verify the hook fires correctly

Cursor Hooks

JSON config + shell scripts. Two types: command and prompt.

Configuration

In .cursor/hooks.json:

{
  "version": 1,
  "hooks": {
    "afterFileEdit": [{ "command": ".cursor/hooks/format.sh" }],
    "beforeShellExecution": [
      { "command": ".cursor/hooks/guard.sh", "matcher": "rm " }
    ]
  }
}

Implementation Types

1. Command-based: Shell script receiving JSON via stdin, returning JSON via stdout. Exit 0 = success, 2 = deny.
2. Prompt-based: LLM-evaluated condition.

   { "type": "prompt", "prompt": "Is this command safe?", "timeout": 10 }
   

Full Reference

For events list, input/output schemas, env vars, matcher values: see hooks_api.md.

Example: Blocking Dangerous Commands

guard.sh:

#!/bin/bash
input=$(cat)
command=$(echo "$input" | jq -r '.command')
if [[ "$command" == *"rm -rf"* ]]; then
  echo '{"permission": "ask", "user_message": "Are you sure?", "agent_message": "rm -rf requires confirmation."}'
else
  echo '{"permission": "allow"}'
fi

Claude Code Hooks

JSON config in settings.json. Four types: command, http, prompt, agent.

Configuration

In .claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": ".claude/hooks/guard.sh",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

Note the nested structure: each event maps to an array of matcher groups, each containing a hooks array.

Implementation Types

1. Command: Shell script. Exit 0 = allow, exit 2 = block (stderr = reason).
2. HTTP: POST event data to URL. Response body parsed as JSON.
3. Prompt: Single-turn LLM evaluation with $ARGUMENTS placeholder.
4. Agent: Multi-turn subagent verification with tool access (up to 50 turns).

Prompt/agent types only available for 8 events: PermissionRequest, PostToolUse, PostToolUseFailure, PreToolUse, Stop, SubagentStop, TaskCompleted, UserPromptSubmit.

Full Reference

For all 18 events, input/output schemas, env vars, exit code behavior per event: see claude_code_hooks_api.md.

Example: Blocking Dangerous Commands

guard.sh (same script works for both Cursor and Claude Code):

#!/bin/bash
input=$(cat)
tool_name=$(echo "$input" | jq -r '.tool_name // empty')
tool_input=$(echo "$input" | jq -r '.tool_input.command // empty')
if [[ "$tool_input" == *"rm -rf"* ]]; then
  echo "Blocked: rm -rf requires manual confirmation" >&2
  exit 2
fi
exit 0

OpenCode Plugins

JS/TS modules returning a Hooks object. No JSON config — all logic is code.

Plugin Structure

import type { Plugin } from "@opencode-ai/plugin"

export default (async ({ project, client, $, directory, worktree }) => {
  return {
    "tool.execute.before": async (input) => {
      // inspect/modify tool arguments
      return input
    },
    event: async (event) => {
      // handle any system event
    },
  }
}) satisfies Plugin

Distribution

• Local: .opencode/plugins/*.ts
• npm: opencode.json → "plugin": ["package-name"]

Full Reference

For all hooks, events, tool() helper, examples: see opencode_plugins_api.md.

Example: Blocking Dangerous Commands

import type { Plugin } from "@opencode-ai/plugin"

export default (async ({ client }) => ({
  "tool.execute.before": async (input) => {
    if (input.tool === "bash" && input.args?.command?.includes("rm -rf")) {
      client.app.log({ body: { level: "warn", message: "Blocked rm -rf" } })
      return { ...input, args: { command: "echo 'Blocked: rm -rf not allowed'" } }
    }
    return input
  },
})) satisfies Plugin

Resources

• hooks_api.md — Cursor: full event list, I/O formats, env vars
• claude_code_hooks_api.md — Claude Code: 18 events, 4 hook types, settings.json format
• opencode_plugins_api.md — OpenCode: plugin API, hooks, events, tool() helper
• assets/hook_template.sh — Bash script template for command-type hooks

Tips

• All IDEs: Paths in hook configs are relative to project root
• Cursor: Use matcher so hooks only fire for relevant commands
• Cursor: Debug via "Hooks" tab in settings or "Hooks" output channel
• Cursor: Set failClosed: true for security-critical hooks
• Claude Code: Use claude --debug or Ctrl+O for verbose hook logging
• Claude Code: Interactive /hooks menu available for managing hooks
• Claude Code: Check stop_hook_active in Stop hooks to prevent infinite loops
• Claude Code: PermissionRequest hooks don't fire in non-interactive mode (-p)
• OpenCode: Plugin dependencies go in .opencode/package.json (Bun auto-installs)
• OpenCode: Plugin tools override built-in tools with matching names